Power drills for drilling masonry are known in which a percussive action is imparted to the drill bit by means of cooperating ratchet plates on a shaft supporting the drill bit and a body of the drill relative to which the shaft rotates. As the cooperating ratchet plates rotate relative to each other, the ratchet plate on the shaft supporting the drill bit is provided with an axial impulse, which is transmitted to the drill bit.
Percussive drills of this type suffer from the drawback that the interaction of a drill bit of such a drill with masonry being drilled generates significant vibrations, which can be detrimental to the health of users of the drill over prolonged periods of use. Such vibrations generally include a high frequency component caused by the vibration of the ratchet plates, typically in the region of 580 Hz, and a low frequency component caused by vibration of a drill bit of the drill in a hole being formed by the drill. Of particular concern are the low frequency components of vibration (typically 10-20 Hz), which are found to cause the most significant long-term health problems. As a result, standards relating to hand-arm vibration use weighting factors to describe the level of vibration likely to cause injury. In particular, a mathematical filter equation is used which emphasises frequencies closest to the 10-20 Hz band, but duration of exposure as well as instantaneous vibration level is taken into account.
It is known to attempt to reduce the vibrations transmitted to the hands of a user of a power tool by providing vibration damping material such as an elastomeric material around at least those parts of the tool housing which are held by the user. However, the stiffness of elastomeric materials is generally too high to significantly attenuate the low frequency component of the vibrations generated by operation of the tool, the low frequency component generally being the most damaging to health.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.